Internal combustion engines often experience different problems associated with varying operating conditions. For example, compression ignition engines, in particular those operating in homogeneous charge compression ignition (HCCI) mode, tend to be susceptible to incomplete combustion at low loads due to low in-cylinder temperatures. It is thus often desired to increase in-cylinder temperatures under these operating conditions by, for example, adding a quantity of internal exhaust residual into the cylinders.
The methods employed to add internal exhaust residual often create additional problems, however. For example, it is common to modify exhaust cams to change the duration in which exhaust valves remain open. This technique may be successful in providing internal exhaust residual at low loads, but it also may be detrimental to the engine during high load operation. Another possible method is to use an engine turbocharger to increase engine back pressure at low loads. For example, a variable geometry turbine (VGT) may be used. The use of a VGT for this purpose, however, also results in additional airflow being created, which in turn reduces the effect of any additional internal exhaust residual.
The present invention is directed to overcoming one or more of the problems as set forth above.